Most important human activities emit greenhouse gases (GHGs). Emissions started to rise dramatically in the 1800s due to the Industrial Revolution and changes in land use. Many greenhouse gas-emitting activities are now essential to the global economy and form a fundamental part of modern life.

Carbon dioxide from the burning of fossil fuels is the largest single source of greenhouse gas emissions from human activities. The supply and use of fossil fuels accounts for about 80 percent of mankind's carbon dioxide (CO₂) emissions, one fifth of the methane (CH₄), and a significant quantity of nitrous oxide (N₂O). It also produces nitrogen oxides (NO_x), hydrocarbons (HCs), and carbon monoxide (CO), which, though not greenhouse gases themselves, influence chemical cycles in the atmosphere that create or destroy other greenhouse gases, such as tropospheric ozone. Meanwhile, fuel-related releases of sulphate aerosols are temporarily masking part of the warming effect of greenhouse gases.

Most emissions associated with energy use result when fossil fuels are burned. Oil, natural gas, and coal (which emits the most carbon per unit of energy supplied) furnish most of the energy used to produce electricity, run automobiles, heat houses, and power factories. If fuel burned completely, the only by-product containing carbon would be carbon dioxide. But combustion is often incomplete, so carbon monoxide and other hydrocarbons are also produced. Nitrous oxide and other nitrogen oxides are produced because fuel combustion causes nitrogen in the fuel or air to combine with oxygen in the air. Sulphur oxides (SO_x) result when sulphur (primarily from coal and heavy fuel oil) combines with oxygen; the resulting sulphate aerosols have a cooling effect on the atmosphere.

Extracting, processing, transporting, and distributing fossil fuels also releases greenhouse gases. These releases can be deliberate, as when natural gas is flared or vented from oil wells, emitting mostly carbon dioxide and methane, respectively. They can also result from accidents, poor maintenance, and small leaks in well heads, pipe fittings, and pipelines. Methane occurring naturally in coal seams as pockets of gas or "dissolved" in the coal itself is released when coal is mined or pulverized. Hydrocarbons enter the atmosphere as a result of oil spills from tanker ships or small losses during the routine fueling of motor vehicles.

Deforestation is the second largest source of carbon dioxide. When forests are cleared for agriculture or development, most of the carbon in the burned or decomposing trees escapes to the atmosphere. However, when new forests are planted the growing trees absorb carbon dioxide, removing it from the atmosphere. Recent net deforestation has occurred mainly in the tropics. There is a great deal of scientific uncertainty about emissions from deforestation and other land-use changes, but it is estimated that from 800 million to 2.4 billion tonnes of carbon are released globally every year.

Producing lime (calcium oxide) to make cement accounts for 3% of CO₂ emissions from industrial sources. Like the CO₂ emitted from fossil fuels, the carbon dioxide released during cement production is derived from limestone and is thus of fossil origin, primarily sea shells and other biomass buried in ancient ocean sediments.

Domesticated animals emit methane. The second-most important greenhouse gas after carbon dioxide, methane is produced by cattle, dairy cows, buffalo, goats, sheep, camels, pigs, and horses. Most livestock-related methane emissions are produced by "enteric fermentation" of food by bacteria and other microbes in the animals' digestive tracts; another source is the decomposition of animal manure. Livestock account for 30% of the methane emissions from human activities.

Rice cultivation also releases methane . . . "Wetland" or "paddy" rice farming produces roughly one-fifth to one-quarter of global methane emissions from human activities. Accounting for over 90 percent of all rice production, wetland rice is grown in fields that are flooded or irrigated for much of the growing season. Bacteria and other micro-organisms in the soil of the flooded rice paddy decompose organic matter and produce methane.

. . . as does the disposal and treatment of garbage and human wastes. When garbage is buried in a landfill, it sooner or later undergoes anaerobic (oxygen-free) decomposition and emits methane (and some carbon dioxide). Unless the gas is captured and used as a fuel, the methane eventually escapes to the atmosphere. This source of methane is more common near cities, where garbage from many homes is brought to a central landfill, than in rural areas where garbage is typically burned or left to decompose in the open air. Methane is also emitted when human waste (sewage) is treated anaerobically, for example in anaerobic ponds or lagoons.

Fertilizer use increases nitrous oxide emissions. The nitrogen contained in many mineral and organic fertilizers and manures enhances the natural processes of nitrification and denitrification that are carried out by bacteria and other microbes in the soil. These processes convert some nitrogen into nitrous oxide. The amount of N₂O emitted for each unit of nitrogen applied to the soil depends on the type and amount of fertilizer, soil conditions, and climate - a complex equation that is not fully understood.

Industry has created a number of long-lived and potent greenhouse gases for specialized uses. Developed in the 1920s, chlorofluorocarbons (CFCs) have been used as propellants in aerosol cans, in the manufacture of plastic foams for cushions and other products, in the cooling coils of refrigerators and air conditioners, as fire extinguishing materials, and as solvents for cleaning. Thanks to the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, atmospheric concentrations of many CFCs are stablizing and expected to decline over the coming decades. Other halocarbons that are being used as ozone-safe replacements for CFCs – notably hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) – contribute to global warming and so are targeted for reduction under the 1997 Kyoto Protocol. The Protocol also targets sulphur hexafluoride (SF₆), used as an electric insulator, heat conductor, and freezing agent; molecule for molecule, its global warming potential is thought to be 23,900 times greater than that of carbon dioxide.